1. Field of the Invention
This invention pertains generally to a device for acoustic beamforming for an underwater vertical array and more specifically to a device for use in shallow water wherein an advanced signal processor utilizing matched beam processing is used to suppress surface generated noise and/or ship radiated noise thereby increasing the detectability of a submerged source emitting only a low noise signal.
2. Description of the Related Art
In underwater acoustics, signal processing is an important tool to enhance the detectability of a signal among unwanted signals (noise) originating from various sources. Since the signal and noise show characteristic directionality in elevation angles, the detection and localization of a submerged source can be improved by adaptive array signal processing which exploits this difference in signal and noise arrival angles. A well known example is adaptive nulling of strong interferences for a horizontal array using adaptive beamforming.
Air-deployed sonobuoy vertical arrays use fixed pre-formed beams to detect signal returns from a passive acoustic source or echo from a submerged object when the signal arrival angle is distinctly different from that of the noise or an interfering source such as a surface ship. With a bottom bounced arrival of a signal the processor uses a delay and sum technique assuming a plane wave arrival for the signal in what is commonly known as the conventional beamformer. The beams are normally focused on the bottom returns of the signal in the first convergence zone of the signal arrivals. A submerged source will be detected by the bottom bounced arrival in deep water as the bottom arrival is relatively clean and free of clutter.
In shallow water, because the signal arrives via many bottom bounced arrivals, the signal energy can be split over several beams. This results in less than theoretical signal gain. The less than optimal signal gain degrades the detection range of the vertical array. There is no clearly defined convergence zone in shallow water. Target range cannot be estimated easily as the difference in multipath arrival angles is comparable or larger than the beam width.
A highly directional noise field can be found in shallow water in summer with a downward refractive sound speed profile. The detection and localization performance of a vertical array against a submerged target in shallow water can be significantly improved if the noise field is directional and the signal arrives in the null (notch) of the noise field. This often happens in a summer environment with the downward refractive sound speed profile in which the surface generated noise field exhibits a notch in the noise vertical directionality distribution at mid (e.g., 500 Hz) frequencies. The noise notch can be weakened or disappear when sound propagation is associated with strong mode coupling.
In a directional noise field when the signal arrives in directions near the noise notch, conventional beamforming can be used to improve the array gain by steering the beams to the shallow arrival angle of the target signal where the noise level is weak. Array gain is a maximum where the incoming wave is confined to one beam. In a multipath environment, the signal arriving on a vertical array can be split into several beams. The signal gain (SG) degradation due to beam splitting often results in a less than theoretical array gain (AG) for conventional beamforming.
Conventional beamforming has been widely used for detection and estimation of a target. In the target look direction, the signals are delayed and summed to yield the highest beam power. In conventional beamforming, the signal gain (SG) of a vertical array is obtained by ##EQU1## where R is the covariance matrix of the signal averaged over many data samples, s is a steering vector with elements of e.sup.-ikzjsin.theta. and p is the data field. The angle bracket denotes average of data over different samples. The k is the wavenumber of the propagating sound wave, z.sub.j, is the depth of the j-th phone and N is the total number of phones in the array. The angle .theta. is measured from the broad side of the array. As it is clear from the above equation, SG will be maximized when .theta. is chosen to match the strongest signal power direction. The noise gain (NG) is similarly defined with a replacement of signal power with a noise power in the above expression.
Matched field processing overcomes the signal gain degradation problem by using a replica field which matches the signal field. When the replical source range and depth coincides with the true source range and depth, the processor yields a theoretical signal gain of 20 log N, where N is the number of sensors. Matched field processing yields a theoretical 10 log N array gain assuming Gaussian white or uncorrelated noise. While noise can be assumed uncorrelated for a horizontal array with proper spacing, it is not in the case of the vertical array. Numerical simulation, theoretical models, and experimental data have all indicated that the surface generated noise is non-isotropic, non-Gaussian, and highly directional in shallow water. For the shallow water case, matched field processing has a limited ability to reject the noise as the correlation of the replica filed with the noise field is not always negligible.
For a vertical array, the matched field processor uses range and depth to discriminate the source from the noise. In principle, the signal would show up as a high level peak at the true source location in the range-depth ambiguity plot where the replica field matches the signal data. Likewise, when the replica field matches the noise data, the local peak will show up in the ambiguity plot indicating the location of the noise sources which should be near the surface. As such, the signal and noise sources are separated by their depths and hence in principle, the signal will be enhanced using depth and range as a discriminator. In practice, this does not work very well in shallow water, because of the extended sidelobes to the source depth. Each surface noise source will produce sidelobes at other depths and ranges other than the true noise source range and depth. Since there are many noise sources spread over the ocean surfaces, the sidelobes from these noise sources accumulate and produce a sizable noise background at the range and depth of the submerged source. This noise background reduces the array gain to below the theoretical array gain.